Long term support of humans in space currently depends on transport of consumables from Earth, with the International Space Station (ISS) being the primary example. Aboard the ISS, oxygen for human respiration is generated by purifying water and then splitting water into oxygen and hydrogen by electrolysis [ref 1]. Meanwhile, carbon dioxide produced by human respiration is scrubbed from the cabin air, concentrated, and emitted from the shuttle [2,3]. Furthermore, food is uploaded from Earth and solid waste is not reused. Therefore, current life support for humans in space depends on open loop technology, which in turn relies on access to supplies from Earth. This open loop technology requires periodic uploads and limits mission duration. Long term human-occupied missions must therefore seek closed loop life support.
The Micro-Ecological Life Support System Alternative (MELiSSA) project, initiated by the European Space Agency in 1989, aims to engineer a closed loop system consisting of five microbial compartments to completely recycle carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous between the compartments and the human-occupied cabin. In addition, a subproject of MELiSSA, named BIORAT, aimed to use a centrifugal planktonic photobioreactor (PBR) to consume CO2 and generate O2 for a simulated human crew of two mice. The 5.6 L photobioreactor successfully supported the two mice for the entire testing period of three weeks [4]. However, the microorganisms in the BIORAT system were cultivated in a liquid medium with a dry weight density on the order of 1 g/L; roughly one kilogram of water is needed to cultivate one gram of biochemically active biomass. The water use intensity of such a system is a potential problem for space exploration where mass is an important constraint. The power consumption of a centrifugal reactor is also a concern.
Current Life Support Technology Aboard the International Space Station.
The oxygen-generating system (OGS) currently aboard the ISS uses electrolysis to split water into hydrogen and oxygen [1,5]. The oxygen is vented into the cabin atmosphere. Some of the hydrogen is used in a Sabatier system to generate water and the rest is released into space. The OGS has a mass of 360 kg and is able to supply oxygen to the astronauts at a variable rate between 2.3 and 9.2 kg/day. The nominal oxygen generation rate is 5.4 kg/day, which is sufficient to support the maximum crew size of six. Moreover, the OGS requires approximately 3.6 kW of electric power, in addition to the 0.8 kW of electric power required to run the urine processing assembly and water processing assembly in series prior to the OGS [5]. Finally, oxygen generation aboard the ISS for a crew of six requires 1480 kg of water to be uploaded to space station from Earth annually [6].
A human exhales approximately 1 kg of carbon dioxide per day, and this CO2 must be constantly removed from the cabin to avoid CO2 toxicity [7]. The current solution to this problem is to first concentrate the exhaled CO2 in a carbon dioxide concentrating assembly (CCA) [3,6]. The CCA consists of a zeolite molecular sieve that preferentially adsorbs CO2. A blower is used to move air through the CCA at an approximate rate of 40 kg/hr. Once concentrated, the carbon dioxide follows one of two paths. Approximately 0.5 kg of each kilogram of concentrated CO2 is fed into a Sabatier reactor. The Sabatier reactor, operated at approximately 225° C., uses a nickel catalyst to enable the reaction [8]CO2+4H2↔CH4+2H2O+heat  (1)
The water from the reaction can be reinserted into the OGS or used for hygienic or cooking purposes, whereas the methane (CH4) is presently vented to space. The zeolite material containing the other half of the CO2 generated in the cabin is heated with electrical heaters to release the CO2, which is emitted into space. The zeolite can then be reused. For normal operation at a crew size of six, the carbon dioxide removal system (CDRA) consumes approximately 840 W of electric power [9]. The CDRA system has a mass of approximately 200 kg.
There are currently no biological food generation capabilities aboard the ISS. Food is uploaded from Earth at an approximate rate of 1200 kg/year [7]. Closed-loop food production in human-occupied space missions must utilize a biological system. Many microorganisms, such as Spirulina platensis, and Spirulina maxima are currently commercially sold as food products and are good candidates for space food production. In addition to food production, a vast array of microbes can be used for remediating waste air and water streams using significantly less energy and mass than conventional space life support systems.
In 1989, the European Space Agency (ESA) initiated the Micro-Ecological Life Support System Alternative (MELiSSA) project [10-13]. The concept of the MELiSSA project is to construct an ecosystem that complements human metabolic processes in order to form a closed loop of the essential elements carbon, nitrogen, oxygen, and hydrogen. To do this, the system includes five compartments, each of which contains a distinct combination of microorganisms capable of specific metabolic pathways. FIG. 1 shows a schematic of the function and proposed microorganism concentration of each of the compartments.
Performance of an interconnected loop (Compartments III and IVa) has been demonstrated at the pilot facility scale [10]. Compartment III included a packed bed biofilm reactor with a total volume of 8 L and containing a co culture of Nitrosomonas europaea and Nitrobacter winogradskyi, which converted ammonium into nitrite and nitrite into nitrate, respectively. Compartment IVa included a 77 L gas-lift photobioreactor containing a planktonic culture of the green algae Spirulina platensis. The authors showed that at a flow rate of 14.5 l/day and an inlet ammonium concentration of 600 mg/l, Compartment III generated nitrate from ammonium at a rate of approximately 8 g/day. Data on the rate of CO2 consumption by Compartment IV was not reported. It should be noted that a gas-lift photobioreactor depends on buoyancy to drive gas bubbles upward through the liquid phase, and this buoyant force does not exist in a microgravity environment. The current goal of the MELiSSA pilot facility is to demonstrate a complete nutrient recycling loop consisting of all five compartments by the year 2015 [14].
BIORAT is a subproject of MELiSSA that integrates Compartments IVa and V (photosynthetic bacteria and crew, respectively) to consume CO2 and to generate O2 for the crew [4].
For breadboard scale demonstration of Compartment Va, Denney et al [4] built a rotating annular photobioreactor that used centrifugal forces for gas exchange and liquid mixing. The reactor is essentially a gas lift reactor that uses centrifugal forces as a substitute for gravity. The photobioreactor was used to cultivate the cyanobacteria Arthospira platensis. The reactor measured 18 cm in diameter and 22 cm in length, for a total working volume of 5.6 L. The crew compartment was simulated by an 11.5 L air tight cage containing a mouse. Food and water were supplied to the mouse and liquid and solid waste were removed as needed and not resupplied.
A control system was implemented to control the rate at which the photobioreactor produced O2 by varying the irradiance incident onto the photobioreactor. The workers operated the BIORAT system for 11 days with a constant oxygen volume fraction in the crew compartment of 0.21, by illuminating the photobioreactor with an irradiance of approximately 20 Watts/m2. The maximum oxygen generation rate of the photobioreactor, tested independently of the crew compartment, was 409 mg O2/hr at an incident irradiance of 115 Watts/m2. At this maximum oxygen generation rate, it would require a photobioreactor volume of approximately 475 L to support one human. The results of this study were reported in the year 2000 and may be the last publication to date on the project.
CO2/O2 Exchange Using a Terrestrial Photobioreactor.
González López et al. engineered and operated a gas-lift photobioreactor containing Anabaena sp. for CO2 consumption [15]. They reported a maximum experimental CO2 consumption rate of 1.45 g CO2 per liter of photobioreactor volume per day at a microorganism concentration of 1 g/L. At this rate, it would require approximately 0.7 kg of biochemically active biomass to support one human. At a biomass concentration of 1 g/l, which is typical of dense planktonic cultures, support of one human would require a planktonic culture volume and mass of approximately 700 L and 700 kg, respectively. Furthermore, because a gas-lift bioreactor cannot function in the absence of gravity, a mechanism similar to the centrifugal pump of the BIORAT photobioreactor would need to be integrated into such a photobioreactor to function in microgravity environments, further increasing the mass of the system.
Ideally, a life support system should process and reuse close to 100 percent of the consumables, especially water, should provide some conversion of CO2 and production of O2, and should use a small amount of power for such purpose. The reprocessing device(s) should have a small footprint, and the reprocessing should occur spontaneously and continuously.
In light of this challenge, we designed and prototyped a novel Surface-Adhering Bioreactor (SABR) in which microorganisms grow in densely packed biofilms (benthic) on a hydrophilic porous substrate rather than in suspension (planktonic). Furthermore, nutrient medium is passively delivered to the biofilm by evaporation and capillary forces. This makes gas and nutrient delivery independent of inertial and gravitational forces, as opposed to the centrifugal air lift photobioreactor component of the BIORAT system. As a proof of concept, we have prototyped a scaled-down SABR prototype to cultivate the cyanobacteria Anabaena variabilis and measure its rate of carbon dioxide consumption. At the same time, we prototyped a conventional photobioreactor of same size and active microorganism loading to evaluate the performance of SABR against the state-of-the-art planktonic PBRs. Finally, we identified ways to improve SABR performance and explored further uses of the SABR system.